add metrics.ConfusionMatrix

[ArneBinder]

from pytorch-ie

context: ArneBinder/pie-core#17 and #172

EDIT: see #188 for a bug fix for this metric.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.88%. Comparing base (e0c089e) to head (501e1f6).
⚠️ Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
+ Coverage   95.82%   95.88%   +0.05%     
==========================================
  Files          67       68       +1     
  Lines        5968     6047      +79     
==========================================
+ Hits         5719     5798      +79     
  Misses        249      249              

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[redspottedbittern]

So, I had some time and added tests already. It is not yet fully done, since I need some feedback. Generally, I added the tests strictly along the lines of Codecov, i.e. for the lines Codecov says tests are missing. So I didn't think about a higher concept